SH3003 Datasheet, PDF (17/30 Pages) Semtech Corporation – MicroBuddy® Support IC for Microcontrollers

SH3003
POWER MANAGEMENT
Application Information (continued)
Low Frequency (LF) Oscillator System
This module provides the 32.768kHz clock to all internal
circuits and to the dedicated output pin, CLK32. If en-
abled, the CLK32 output continues normal operations
when VDD is absent and backup power is available.
When the power is first applied to the SH3003, the RC
oscillator takes over. It supplies the 32.768 kHz clock for
start-up and initialization. However, if the CLKSEL pin is
set high, then the crystal oscillator is enabled. Once the
crystal has started and stabilized, the internal 32.768 Hz
clock switches to the very accurate crystal frequency; see
Figure 5.
Just like the VBO value for the Reset circuit, the default
calibration values for the RC oscillator are loaded on pow-
er-up from the factory-programmed non-volatile memo-
ry. They can be re-programmed at any time or they can
be permanently protected from any changes by setting
the Lock flag or a write-protect flag. Factory calibration
brings the frequency of the RC oscillator within ± % of the
32.768kHz for the internal reference resistor, and ±2% for
the external 1MΩ 1% resistor, over the entire temperature
and supply voltage range.
The frequency of the RC oscillator can be tuned or mod-
ulated by varying the external reference resistor, which
should be located as close as possible to RREF, pin 9.
The crystal oscillator has the useful feature of adjustable
load capacitors. It permits tuning of the circuit for initial
tolerance of the crystal (often ±20ppm) as well as an ad-
justment for the required load capacitance (with possible
variations from the PCB layout). While the oscillator was
designed for a crystal with a nominal load capacitance of
12.5pF, the circuit accommodates any value from ~7pF to
22pF (depending on parasitics of the layout). All of these
corrections can be performed when the part is already
installed on the PCB, in the actual circuit.
The default value for load capacitance (12.5pF) loaded on
power-up from the factory-programmed nonvolatile mem-
ory can be re-programmed at any time (following a secure
process of unlocking the load capacitance value register
and immediately writing a new setting), or it can be com-
pletely protected from any changes by a permanent write-
protect flag. This adjustment can set the frequency of the
crystal oscillator to within ±4ppm of the ideal value. As
a reference, a typical 32.768kHz crystal changes its fre-
quency 4ppm for a 10°C change in temperature.
Since the temperature characteristics of crystals are well
known and stable, the host processor is free to implement
an algorithm for temperature compensation of the crystal
oscillator using the adjustable load capacitors, with re-
sulting accuracy of ±4ppm over the entire temperature
range.
32.768 kHz
Crystal
12.5 pF Load
Capacitance
VREG
X-TAL
RC
XIN
5
XOUT
6
CLKSEL
7
RREF
9
External
Reference
Resistor
1 M 1% or
variable
VSS
Lock / Unlock
8
Logic
4-bit Value
VSS
1
X-tal stable?
INTERNAL
RREF ON
RC
Oscillator
Internal RREF
6-bit Value 4-bit Value Lock Logic
Figure 5. Simplified LF Oscillator System
CLK32 ON
Internal
32.768
kHz Clock
From / To
Serial I/O
CLK32
13
©2006 Semtech Corp.
17
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